Timing Is Everything

You don’t have to rely on guesswork or intuition to catch sight of the ISS as it passes over you. To determine exactly where and when to see it or any other specific satellite, you can download a computer program that calculates the particulars using two-line elements (TLEs). These are strings of numbers that define the satellite’s motion mathematically, using precise values for its orbital inclination, the number of orbits completed in a 24-hour period, and so forth. Satellites at relatively low altitudes are susceptible to atmospheric drag and undergo slight changes in motion caused by Earth’s gravity, and for these you’ll need to obtain updated sets of TLEs every few weeks to keep the predictions accurate.

I’m sure you’re thinking, “There must be an easier way!” And there is, at least for the International Space Station. You can customized predictions for your location right here on SkyandTelescope.com. Just use our satellite tracker. NASA also provides ISS viewing predictions for hundreds of cities worldwide.

To spot the spacecraft, go outside a minute or two before the indicated time and look in the compass direction where the ISS is predicted to appear. Pick a spot without trees or tall buildings to block your view in the appropriate direction, especially if the maximum elevation is less than 30°. Don’t fixate on a particular point in the sky, as often the ISS is too faint to pick up at the very beginning of its pass.

I like to challenge kids or friends at outdoor parties to see who’ll be the first to catch sight of the ISS as it rises up into the sky. Or try estimating its brightness by making comparisons to other bright stars or planets. You can try following it with binoculars, but they’re not powerful enough to detect its shape. And while a telescope can resolve crude details, you’ll find that it’s very difficult to follow the spacecraft smoothly.

The most popular website for tracking satellites is heavens-above.com, which offers easily customized sighting predictions for the ISS and other bright spacecraft. You can display or print a star chart showing the satellite’s track across the sky, look up sightings for past dates (very handy if you saw a bright satellite last night and want to know what it was), and create predictions for the fascinating phenomenon of “Iridium flares.”

Sparks in the Dark

Of the thousands of objects now orbiting Earth, the nearly 100 spacecraft belonging to the Iridium series really stand apart — at least from a skywatcher’s point of view. These communications platforms skim the upper-most, most rarefied region of the atmosphere around 500 miles up. They travel in six steeply inclined orbital planes that nearly pass over the North and South Poles, making them observable from anywhere on the planet.

What makes the Iridium satellites special is that each has three extremely shiny, door-size antenna arrays. These act like giant mirrors, reflecting sunlight toward the ground and causing brief but brilliant flares that can momentarily outshine any star or planet. (In astronomical parlance, the planet Venus can be as bright as magnitude –4½ but an Iridium flare can be –8, which is 25 times brighter.)

For pure spectacle, nothing beats a bright Iridium flare, and you’ll find predictions for them on the Heavens-Above website. They’re easy to photograph, too. Virtually any camera, whether film or digital, can capture a bright flare as long as it can take a time exposure at least 15 or 20 seconds long. Use film (or an ISO setting) of 100 or 200, set the lens opening to f/2.8 or faster, and mount the camera on a sturdy tripod.

The only challenge is making sure your camera is pointing at the right spot in the sky. Iridium predictions are given in angles of azimuth and altitude. The azimuth is measured around the horizon starting at true north (the direction of Polaris), increasing along the horizon toward east(90°), south (180°), west (270°), and back to north (0° or 360°). Altitude runs from 0° at the horizon to 90° directly overhead. Your fist, held at the end of an outstretched arm, is about 10° across — a useful tool for estimating angles. Iridium flares leave streaks that are usually less than 10° long, so you'll have plenty of room for small pointing errors if you're using a normal or wide-angle camera lens.

Do-it-Yourself Predictions

Want to track down the Hubble Space Telescope or a tumbling rocket body in space? Computer software to create customized viewing predictions is available from T. S. Kelso (www.celestrak.com/software)and the Visual Satellite Observer’s Home Page (www.satobs.org/orbsoft.html).You can download two-line elements for thousands of different satellites at Kelso’s website, as well as from www.space-track.org.